May 31, 2020

15.05.11: Gravity Probe B Scores Another One for Einstein.

One of the Gravity Probe B Spheroids…(Credit: NASA/Don Harley)

A mission decades in the making has come to fruition. Recently, scientists have announced the results of the Gravity Probe B experiment. This mission was conceived way back in 1963 and had to await the birth of entirely new technologies before even reaching orbit. The idea was simple; place a set of ultra-precisely machined spheres in orbit, set them to spinning as a star tracker keeps tabs on your orientation, and watch and see if the effects of frame dragging and the warping of space-time are apparent. Simple in theory, but first, much larger effects, namely that of solar and atmospheric drag would have to be accounted for and counter-acted. And the star tracker would have to be ultra-precise, as it stared off at the star IM Pegasi from its polar orbit vantage point. Yes, the effects from the measurements of frame dragging by even a large mass such as the earth are tiny… enough so that they would be totally swapped by mundane terrestrial phenomena such as earthquakes and the neighbors Hummer driving by playing Too Short on his tricked-out stereo. But in the end, evidence for frame dragging was detected by the onboard gyroscopes monitoring their spin orientation to an unprecedented degree of precision. These Niobium coated globes used in the gyros have been touted as the “most precise spheres ever made…” The required degree of accuracy was <0.5 milliarcseconds per year, and the predicted drift for the geodetic and frame-dragging effect was 6.61 & 0.041 arc seconds per year respectively… (try getting THAT kind of accuracy out of your GOTO scope!) The data results jibe nicely with the predictions made by Einstein way back in 1916.

The fact that gravity warps space-time was first demonstrated during an eclipse expedition in 1919 and was one of the first great proofs of general relativity. Since the beginning of the Gravity Probe B program in the 1960’s (!) over a hundred doctoral students have cut their intellectual teeth on the project, including a few you might have heard of, such as Sally Ride (the 1st US woman in space) and Nobel prize winner Eric Cornell. But perhaps the greatest legacy that Gravity Probe B will have is not the basement Einstein naysayers that its six year mission has put to rest, but the very legacy of the new technology that it pioneered. From ultra-high precision location and machining to drag-free satellites, Gravity Probe B has led the way. Thank Einstein and relativity that your GPS is accurate on the way to the local store on that all essential beer run; the signal that you receive would be off by several dozen meters in relativity weren’t taken into account.

But I think the real take away lesson from Gravity Probe B is how science works; rather than simply saying “good enough,” we’re still out there looking to see if a hallowed theory breaks down, even by a big shot such as Einstein. This is good science in action… perhaps there is a point where relativity will be in need an over-haul; Gravity Probe B could have had a result where physics went off in some new and exciting direction. Score another one for little Al from Princeton!

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